1. Field
The present invention relates to a liquid crystal display element that drives cholesteric liquid crystal to display an image, a method of driving the same, and an electronic paper using the same.
2. Description of the Related Art
In recent years, electronic papers have been actively developed. For example, a reflective display element using a cholesteric-phase liquid crystal composition (hereinafter, referred to as cholesteric liquid crystal) has been used for the electronic paper. The reflective display element using the cholesteric liquid crystal has a memory display function of semipermanently displaying an image even when no power is supplied, and has good display characteristics, such as a clear color display characteristic, a high contrast characteristic, and a high-resolution display characteristic.
Since the reflective display element using the cholesteric liquid crystal has such display characteristics, it can be appropriately used as a display unit of an electronic paper, a mobile terminal, or a portable device, such as an IC card.
Further, the reflective display element using the cholesteric liquid crystal can be used as an outdoor advertising board that uses the memory display function to display a large image, such as an advertisement image, out of doors for a long time without consuming power and displays another image after a predetermined time has elapsed. See Patent document 1: Japanese laid-open patent application No. 2001-100182.
The reflective display element using the cholesteric liquid crystal performs grayscale display by changing the voltage value or the pulse width of a pulse voltage applied to the liquid crystal.
However, response characteristics of the liquid crystal depend on the temperature. When voltage application conditions (the voltage value and the pulse width of a pulse voltage) that have been set to perform optimum grayscale display at room temperature are applied to a display process at high temperature without any change, a deep image is displayed since the response characteristics of the liquid crystal at high temperature are higher than that at room temperature.
Therefore, it is necessary to adjust the voltage application conditions to the liquid crystal according to a temperature variation. The temperature compensation of the response characteristics of the liquid crystal is performed by a pulse width modulation method rather than a voltage modulation method in order to reduce costs. The pulse width modulation method applies a pulse voltage with a pulse width that is shorter than that at room temperature to the liquid crystal at high temperature, without changing the value of the voltage applied to the liquid crystal. Therefore, the transmission rate of image data transmitted from a processor to a driver control circuit of a liquid crystal display element at high temperature needs to be higher than that at room temperature. In this case, it is necessary to provide a high-speed transmission driver and receiver in correspondence with an increase in the processing load of the processor. As a result, the manufacturing costs of an apparatus increase.